This invention relates to driveline torsion damping mechanisms operable over the entire operational range of a driveline. More specifically, the invention relates to such mechanisms for vehicle drivelines.
It is well-known that the speed of an Otto or Diesel engine output or crankshaft varies even during so-called steady-state operation of the engine, i.e., the shaft continuously accelerates and decelerates about the average speed of the shaft. The accelerations and decelerations are, of course for the most part, a result of power pulses from engine cylinders. The pulses may be of uniform frequency and amplitude when cylinder charge density, air/fuel ratio, and ignition are uniform. However, such uniformity does not always occur, thereby producing pulses which vary substantially in frequency and amplitude. Whether uniform or not, the pulses, which are herein referred to torsionals, are transmitted through vehicle drivelines and to passengers in vehicles. The torsionals, which manifest themselves as vibrations, are detrimental to drivelines and derogate passenger-ride quality. Further, when an engine is abruptly accelerated and/or decelerated by accelerator pedal movement, torque pulses ring through the driveline and also derogate ride quality, such pulses are herein also referred to as torsionals.
Since the inception of the automobile, many torsion damping devices or schemes have been proposed and used to isolate and dampen driveline torsionals. For example, master clutches, used in combination with mechanical transmissions, have long employed springs and secondary mechanical friction devices to respectively isolate and dampen torsionals. Typically, torsionals are isolated or absorbed by a plurality of circumferentially spaced, coil springs disposed in parallel between the master clutch primary friction input and splined output. Damping is provided by secondary mechanical friction surfaces disposed in parallel with the springs and biased together with a predetermined force. Damping occurs when the amplitude of the torsionals exceeds the breakaway or slip torque of the secondary friction surfaces. With this arrangement, portions of the torsionals less than the slip torque of the secondary friction surfaces are transmitted directly through the clutch without flexing or isolation by the springs, i.e., the arrangement provides neither torsion isolation nor damping. If the slip torque of the secondary friction surfaces is reduced by design or wear of the secondary surfaces, damping is reduced. Further, any portions of the torsionals greater than the spring energy absorption or storage capacity are also transmitted directly through the clutch. If the spring rate is increased to provide greater storage capacity and prevent spring collapse, the springs transmit lesser amplitude torsionals directly through with little or no effective isolation or absorption of the torsionals.
To increase the operational spring range and storage capacity of a torsion damping assembly, Wemp in U.S. Pat. No. 1,978,922, proposed using a low spring rate torsion sleeve capable of flexing substantially more than the coil springs used with master clutches. This arrangement, like the master clutch arrangement, also employs secondary mechanical friction surfaces disposed in parallel and biased together with a predetermined force to provide damping. Hence, the Wemp arrangement also fails to provide isolation and damping of torsionals below the slip or breakaway torque of the secondary friction surfaces. The Wemp arrangement is also underdamped if the slip or breakaway torque of the secondary friction surfaces is reduced.
The advent of torque converter-type automatic transissions ushered in a whole new perception of torsion damping and, of course, passenger ride quality. While torque converters have many advantages, one being torsional damping, they embody inherent slip and, therefore, inherent losses in vehicle fuel economy. In an effort to minimize this slippage and thereby optimize or improve fuel economy, various efforts have been made to bypass the torque converter with some manner of direct drive which is typically brought into play when a vehicle is operating in the higher speed ratios of the transmission. While these direct-drive bypass arrangements have resulted in fuel economy improvement, they have also brought back driveline vibration with resultant derogation in the vehicle ride quality that passengers have become accustomed to over the years. The direct drive bypasses, for the most part, have been in the form of master type friction clutches with torsion damping devices similar to the previously mentioned devices. One example of such a bypass is disclosed in U.S. Pat. No. 4,194,604. Two further examples of bypass drives are disclosed in U.S. Pat. Nos. 3,977,502 and 4,317,510. In the '502 patent, the master type clutch engagement force is such that the clutch primary friction surface continuously slips or slips in response to torsionals above a predetermined amount. This arrangement is difficult to control since the engagement force must vary with driveline torque. In the '510 patent, the master clutch incorporates a viscous coupling which continuously slips to dampen torsionals in a manner analogous to the continously slipping clutch in the '502 patent. With the arrangement in both of these patents, substantially all of the energy from the engine to the transmission must be transmitted across slipping surfaces; hence, both arrangements generate substantial amounts of heat and, of course, losses in the form of fuel economy. A third bypass arrangement, as disclosed in U.S. Pat. No. 4,138,003, includes the master type clutch in combination with low-rate torsion isolation springs which may be of the helical torsion type or of the torsion bar type analogous to the arrangement disclosed in previously mentioned U.S. Pat. No. 1,978,922. It is also known to use flat torsion springs in vibration dampers, as diclosed in U.S. Pat. No. 4,181,208.
Previously mentioned copending U.S. application Ser. No. 564,537 and entitled Torsion Damping Assembly discloses the use of a viscous coupling in lieu of the secondary mechanical friction surfaces used in the prior art torsional damping mechanisms. Since the clutching medium therein is a viscous liquid, breakaway torque associated with the mechanical friction surfaces is eliminated. Hence, the coupling provides damping over the entire operational or torque range of the assembly. The coupling has a constant damping factor and has provided excellent results in tested vehicles. The torsion damping mechanism disclosed herein improves the assembly of the copending application by varying the damping factor of the viscous coupling therein.